Touch screens are well-established computer input devices. Uses of touch screens include point-of-sale applications like cash registers at fast-food restaurants, point-of-information applications such as department store information kiosks, and ticketing applications such as airline-ticket kiosks. As touch screen technologies mature, the range of applications increases including into portable handheld devices.
Commercially available touch screens utilize a variety of different touch detection mechanisms. One type of touch detection mechanism that appears to be well suited for handheld applications is based on a dielectric capacitive touch screen. Such systems are often referred to as projected capacitive touch screens since the detection mechanism involves projecting electric fields through a dielectric layer. “Projected capacitive” touchscreen is in contrast to a “surface capacitive” touchscreen that has a single sensing electrode covering the entire touch area. As used herein, “projected capacitive touchscreen” generalizes to any capacitive touchscreen with a plurality of sensing electrodes in the touch sensitive area. Projected capacitive touch screens provide better optical transmission and clarity than resistive touchscreens often used in handheld systems and yet provide good sensitivity to finger touches.
In one type of conventional projected capacitive sensors, one layer of horizontally oriented transparent electrodes plus a separate layer of vertically oriented transparent electrodes are required. The transparent electrodes are typically formed from thin films of ITO (indium tin oxide). Such projected capacitive sensors can provide excellent touch performance but manufacturing two layers of transparent electrodes adds cost.
Of commercial interest are projected capacitive sensor designs in which both horizontal and vertical coordinates are determined using a single plane of transparent electrodes. Such systems (for example as it FIG. 1) may provide touch performance of sufficient quality for many applications at a reduced cost.
FIG. 1 illustrates a conventional electrode configuration that has been proposed for a projected capacitive touch screen. In FIG. 1, electrodes 101-105 are formed of triangular regions of ITO or other transparent conductive film. Between electrodes 101-105 are insulating gaps often referred to as “deletion lines” as cost-effective manufacturing often starts with a glass substrate with a uniform ITO coating from which selected regions of ITO are removed to form the pattern of triangular electrodes 101-105. Each electrode 101-105 is electrically connected to the associated electronics, such as through metal traces indicated to the left of FIG. 1.
However, conventional capacitive touch screens have experienced certain limitations, particularly in small touch screens such as in handheld devices. For example, the conventional projected capacitive touch screens experience an effect referred to as edge deceleration. FIG. 2 illustrates the edge deceleration effect that may be experienced by a projected capacitive touch screen with a small touch sensitive area 110. FIG. 2 illustrates a finger moving along a line or trajectory 128 while in contact with the touch screen surface. Finger contact areas 122, 124 and 126 correspond to three different times during the finger motion along trajectory 128. For a well designed projected capacitive system, reconstructed (X,Y) touch coordinates 132 and 134 will be at the center of the corresponding touch contact areas 122 and 124. However, for touch contact area 126, the finger has partially left the touch sensitive area 110 and only the portion of the touch within touch sensitive area 110 is detected. This may result in reconstructed (X,Y) touch coordinates 136 which are not at the center of the touch contact area 126. Instead, the reconstructed (X,Y) touch coordinates are at an “effective center” which is at the center of the on-screen portion or detected touch contact area within the touch sensitive area 110. The trajectory 130 of reconstructed (X,Y) touch coordinates 136 may deviate from the true finger trajectory 128 near the edge of the touch area producing an artificial deceleration of the measured finger motion velocity component that is perpendicular to the edge of the touch sensitive area 110.
A need remains for electrode configurations and geometries that compensate for various effects such as the deceleration effect.